Nature vs Nurture
by Dayja
Summary: A mad scientist has attempted to switch John and Sherlock's minds! Except it doesn't work. Or does it? If Sherlock remembers being Sherlock, and John remembers being John, then does that mean they are Sherlock and John? Or are people more than their memories? What if their minds switched bodies but their memories didn't? Then who would be Sherlock, and who would be John?


Warnings: Occasional descriptions of violence, character death, and a very brief instance of creepiness with non-con undertones.

Disclaimer: I do not own, am not associated with, make no money from Sherlock.

**1**

"You're too late!" the man screamed in true mad scientist style. "I did it! I actually did it! I have switched the minds of these two men with my wonderful machine!"

"Fried them, more likely," Donovan muttered as she led him away, only allowing a brief glance of concern to show for the two men strapped to the machine. Neither man was awake and conscious; the machine had certainly done something to them. Mostly it had just flashed a lot of lights, like some weird futuristic device dreamed up in the eighties, complete with metal bowls strapped over their heads.

Lestrade had left the scientist to her while he worked on undoing the restraints to their two wayward consultants. His face was almost pained as he worked. That was the trouble with being a really good detective; he had a really fertile imagination for considering all the ways this mysterious machine might have destroyed his friends. Having actually had their minds switched was only the start of the list, and not the most likely. Having literally had their brains fried was much more likely. The only thing anyone was certain of was that they were both still alive.

Then, just as the paramedics were getting ready to take them, they both opened their eyes.

"Well, thank goodness for that," Lestrade said, feeling almost giddy with relief, "Now just tell me you didn't actually change bodies."

"Of course we didn't," John Watson answered. He was staring at Sherlock, his eyes roving over his body. Sherlock was doing much the same to John. They almost seemed to be communicating, all the sorts of things close friends might say in this sort of situation, but without a need for the actual words: _are you hurt? Are you okay? Of course I'm fine, don't worry. I'm glad you're safe_. Neither of them were screaming or saying 'what am I doing outside my body?'. So far, they seemed completely normal. Mostly. There was an unfamiliar expression hovering about their faces. Something like confusion.

"Right," Lestrade said, "Well, let's just get you to hospital, then, and make sure of things?"

"No," they said, eerily in sync.

"Yes," Lestrade insisted. "You just had your brains messed with by an unknown machine and were unconscious for several minutes. You need to be checked out."

"I'm fine, Greg," John answered, his voice scathing and full of annoyance. "I'm a doctor; I'd know if something were wrong."

"Yeah?" Lestrade asked. "And what about Sherlock? Are you willing to risk his health?"

John's expression faltered, but it wasn't him who finally answered.

"Maybe we should let them look us over," Sherlock said, still scanning over John's body with concern. "I…I don't know…I didn't learn everything about brains, my mind palace doesn't have the right information to be certain…I don't know enough. I don't know enough to keep you safe."

"And you can't trust my knowledge?" John demanded, but without any heat behind the statement. He looked resigned now, had, in fact, from the moment Lestrade had mentioned Sherlock's health.

"Of course I can," Sherlock answered, "Half my empty data folders have 'consult John for further details' written in them. But it's your brain that I have questions about. What if you're compromised?"

"They do?" John asked, sounding surprised and pleased.

"Of course they do. Why should I clutter up my mind palace when I have you?"

In the end, they both went. All the tests came back as normal. Their brain patterns were the same; their memories seemed unaffected.

That didn't stop Lestrade from feeling like something was different about them. He just couldn't figure out what.

**2**

Sherlock Holmes opened his eyes. He was William Sherlock Scott Holmes. He knew this, because he remembered it. He remembered this bedroom and waking up in it before. He remembered a lifetime of being Sherlock. His mind palace held rooms and rooms of data. When he looked at the world, he was able to observe.

At the hospital, he had looked at the neurological doctor and he knew that he had a two year old son, that he was faithful to his wife despite the attempts of two different nurses, and that despite otherwise being a good husband, father, and doctor, he also had issues with gambling.

The doctor was also very annoying with his questions. Sherlock didn't want to be in the hospital anyway, and all he wanted to know for sure was that there was nothing wrong with John and that his own brain wasn't going to suddenly stop working. After all, his intelligence and his ability to observe were pretty much the only things he had going for him. He remembered a long, friendless lifetime that reinforced this.

Sherlock also remembered past experiences wherein he'd push away annoying people by deducing their faults out loud. Sometimes this was done on purpose, sometimes things just came out.

He didn't do that this time. The facts were there, and the annoyance was there, but the tactic felt…wrong for him. It felt like…like when he had to act out a role for an investigation. He was good at acting, and to do it properly, one had to understand the persona you wanted to show. His new persona was a person who remembered being Sherlock Holmes, and observed like Sherlock Holmes but…well, there was a but, a big one, except he wasn't entirely certain what it was.

Was it that he hadn't wanted to hurt the doctor? No, it wasn't quite that. His emotional capacity hadn't changed. Why should he care about someone he didn't know? It was more like he wanted the man to like him. Sherlock could just about remember feeling that before. He had felt it, to a small degree, when he met John Watson, but that had been tied up in his desire to keep a flatmate for longer than a day and with wanting to impress him. He used to want people to like him, hadn't he? But it never worked; he knew that. It never worked and he had given up trying unless he needed to charm someone for a case.

He didn't need to charm their doctor, but he did want him gone. Charming him was one way to achieve that, even if it was not his usual tactic. So he had flattered the man for resisting those nurses and proved his undiminished intellectual capacity and observation skills by explaining, and skipped over the gambling completely. And it had worked, more or less. The man was convinced his brain was unharmed, and he sent him on his way with a smile.

Sherlock knew that his name was Sherlock Holmes, and he knew how to observe, and he knew the names of his brother and his parents and his coworkers and his friends, and he had a mind palace full of facts.

He also knew that something was different. Something was wrong.

Because he was wearing Sherlock's body, and living Sherlock's life, and had Sherlock's memories and skills and name. But it felt like it didn't belong to him.

He got up and got dressed in Sherlock's clothes that felt familiar and comfortable and wrong all at the same time, and he went out into the sitting room. After a moment of feeling unsettled and odd, he finally tried to do what he usually did when his thoughts and emotions became tangled.

He picked up his violin and played a song. His fingers remembered how to move. His brain remembered the music as though he had it on a stand in front of him. He remembered loving this piece.

He played it absolutely perfectly and without an ounce of passion. His mind felt as confused and as cluttered as before. Didn't music usually help? Didn't he love that song?

"Oh, that was lovely," said John as entered the room. Sherlock found himself smiling at the praise. This, at least, was something that the memory Sherlock and the new Sherlock agreed upon; they liked pleasing John. He played another piece, one he remembered John liking before. Somehow, it came more naturally this time, perhaps because he had John to play for.

John smiled, and the feeling of wrongness eased. He was Sherlock Holmes and his friend was John Watson. For the moment, that was enough.

**3**

The man who knew his name to be John Hamish Watson remembered his life in boxes.

The boxes weren't a conscious effort, not like a mind palace, and nor were they literal boxes that he could visualize. It was simply that John Watson's past held moments of pain, moments of savagery, and moments of intense sadness. And when he needed to overcome pain, or to be savage, or to indulge in sadness, then he had these moments to guide him. And when he needed to be plain, ordinary John who didn't go around killing people or mourning the dead, then he could shut the lid and get on with life. Not literally, because there were no literal boxes in his brain and no literal lids, but that was, more or less, what he did all the same.

Ever since that first awakening, after the machine, the man who remembered being John Watson found all his boxes' lids had cracked open and bled into one another.

Of course, he didn't literally imagine boxes so he didn't literally imagine that something had happened to those boxes. All he knew was that he felt completely out of sorts and raw. The war was closer to the surface than it had been since he had lost his limp. In fact, he had felt twinges of pain in his leg, off and on, since the machine as well. All of his emotions felt close to the surface, jumbled and confused. He found himself snapping at Greg, even though he knew the man was just concerned about them. He snapped at the doctors, after they kept trying to treat him like a patient and forgetting he was a doctor too. If there was a way to snap at one's own brain, then he did that too.

And then he heard Sherlock playing, and just like that, the lids snapped shut and his very soul seemed to settle.

"Oh, that was lovely," he said when it finished, and Sherlock smiled. There, at least, was one person who he didn't feel out of sorts with; he still loved making him smile. No matter how out of sorts or raw he was in every other way, John Watson knew that Sherlock Holmes was his friend.

**4**

Lestrade waited a full week before he gave in and called Sherlock in on a case. Like usual, Sherlock and John came together.

Not like usual, John was half a step ahead of the detective. It wasn't that John looked more excited than Sherlock or that he was rushing. Both men's expressions of interest were about the same as they always were. But Sherlock was still half a step behind his blogger instead of three steps ahead.

Lestrade watched both men closely as they navigated the crime scene. Not that he didn't usually watch his consultants carefully, particularly considering Sherlock's tendency to run off, steal evidence, and his general reluctance to explain himself. But this time he wasn't concerned about his crime scene, he was concerned about his consultants.

There was still something odd about them.

It wasn't that Sherlock wasn't able to deduce all of a sudden; he clearly still could. It was that normally Sherlock flew. He glided about the scene, he billowed, larger than life, and he shone. This Sherlock crept gently. He unobtrusively slid into the scene, not ruffling a single investigator, and his eyes took in everything.

John didn't crash about; he wasn't clumsy or in the way or rude, but somehow, something about the way he stood, the way he walked, even the way he took in the scene made it impossible for him to blend into the background. When he crossed paths with someone going in the opposite direction, it was not John who stepped to the side to let them pass.

And then Sherlock spoke out loud, and not just to John but to everyone. He explained everything; every deduction, every theory, every bit of reasoning they might use. He explained, and he did it without throwing out insults.

"Fantastic!" John told him, and that was perfectly natural for John to have said, and despite the oddity in their behavior, Lestrade could almost have relaxed. Besides, he now had a lot more information to process thanks to Sherlock's observations. Everything was going well.

And then John got in a fight with Donovan.

By this point, Lestrade was no longer watching the two men like a hawk and had gone to talk to the forensics team and make sure everyone was on the same page, especially since he habitually kept the forensics team as far from Sherlock as possible to avoid conflicts. So the first thing he knew about it was a loud, angry voice shouting from near the police tape.

"And did you ever think!" John's voice said, loud and angry, "That maybe he solves your little puzzles because he likes to help?!"

"What are you on about?" Donovan demanded right back, not shouting but her voice still carrying all the same. "The Freak does it because he likes the puzzles. He doesn't care about the people."

"And so what if he cares?" John demands. "How does caring about people help?! If he's really the psychopath you think he is, then there are a hundred more destructive ways he could be using his genius. But he chose this way! He chose the way that does good! And he does it for you, without pay!"

"Er…John," Sherlock mumbled, eyes towards the ground. "It's really alright, John."

"You know, you're right," Donovan says, much quieter now, but Lestrade had started for them during the shouting, so he's close enough to hear her when she says, "Maybe it isn't the Freak who's the psychopath. Maybe it's you we should've been watching all along."

So Lestrade is close enough to hear her but not quite close enough to stop Sherlock Holmes when he suddenly moves from that oddly unobtrusive and hunched position to slamming Donovan against the building, movements precise and calculated and controlled.

"Sherlock!" Lestrade and John bark out at the same time. Sherlock doesn't blink or shake himself and startle out of it. He looks at Donovan, glares, and then deigns to let her go. She staggers against the wall with a frightened gasp, then flees the scene as fast as possible without actually breaking into a run.

John sighs.

"Well, that's convinced her we aren't psychopaths," John said, his voice far too jovial for the situation.

"You do know she's going to press charges, right?" Lestrade demands. "What was that about, Sherlock?"

"Then I should press charges for verbal harassment," Sherlock answered.

"Yeah, good luck with that one," Lestrade answered, almost laughing at the sheer hypocrisy of that statement considering the number of insults Sherlock usually wielded against anyone he deemed inferior to himself. Which was everyone.

"She is a professional," John pointed out. "She shouldn't be able to talk to civilian consultants like that. Calling Sherlock a freak shouldn't be allowed."

"She can call me a freak all she wants," Sherlock said, frowning. "I'm used to it anyway. But she has no right to say that about John. It isn't true."

"It isn't true about you being a freak either!" John answered.

Lestrade just looked at them. They both looked stressed and the sense that there was something wrong with them was stronger than ever. Maybe it had been too soon to call them to a case. Maybe that machine had done something. He didn't know what, but something was different. Sherlock didn't usually act like that, not unless…

"Sherlock…" Lestrade began slowly, not quite sure how hard he needed to push his suspicions. Sherlock jumped to the correct conclusions anyay.

"I'm not using," he said. It wasn't a snarl like it usually was either, just matter of fact.

"What?" John said, and then, "No, he's really not. I'd have noticed. We haven't even really been out since…well. I'd have noticed." Then, at Lestrade's skeptical look, "I am a doctor, you know. And I've dealt with addicts before. I'd have noticed."

"Well, something is off about you two," Lestrade said. "Ever since that mad scientist's machine."

Sherlock and John looked at each other, one of those looks that shared words that Lestrade didn't understand. Then they looked at Lestrade again.

"Maybe…" Sherlock said after a moment, "Maybe we should…try to learn more about that machine?"

The agreement didn't make Lestrade feel any better. It was practically an admission that the machine had done something to them. Something was wrong.

**5**

They didn't get to see the actual machine which was still being studied, but they did have the reports sent to Baker Street, including what Dr. Patrick Cliff had intended it for.

"He said it was supposed to switch minds into different bodies," Sherlock said thoughtfully. "He came up with the idea after his wife got sick. That we could switch the minds of those with ailing bodies with criminals. Of course no one would fund such an insane endeavor, and ultimately his wife died before he could complete the project, but he did it anyway. And it didn't seem to work. I'm Still Sherlock Holmes and he's still John Watson."

"But you aren't," Lestrade answered. "Not to people who know you. Something's different. Do you think…do you think there's brain damage?"

It was an awkward question to pose to the people who might, in fact, have that brain damage, but it had to be asked. Lestrade had seen enough injuries in his line of work to be familiar with the possible repercussions of a blow to the head. Changes in personality were definitely a possibility.

"It's a bit like depersonalization disorder," John suggested, "Except I don't have any sensation of floating or that I'm actually outside my body, more…like…"

"Like the body I'm in doesn't fit quite right," Sherlock suggested.

"Like that," John agreed. He sounded almost relieved. "You feel it too? Since the machine?"

So this was apparently the first time they'd admitted it to each other, Lestrade realized. A full week of feeling like this and they hadn't even talked about it.

"It's better when we're together," Sherlock said. "Because I still…because you are you and I am me and that's what matters and…"

"Memory." John asked, not so much in response to what Sherlock had said as he blurted it out suddenly in sudden realization.

"Memory?" Sherlock asked. John nodded, looking confident once again.

"They scanned our brains, didn't they, and there was nothing different from scans they've done in the past. We have those on file, of course, considering how often we get bashed in the head. Well, brains are like fingerprints, a bit. They're unique. And brains hold our memory physically. If the machine had switched our memories, then it'd show up when they scanned our brain. But if the machine did manage to switch some aspect of us…not our memories but our…our consciousness, our…minds…"

"Our souls?" Sherlock suggested with a faint quirk to his lips, as though he were only half serious.

"I studied medicine, not spirituality," John answered, and then paused. "Or at least, this body studied medicine and remembers it for me. So perhaps the machine did switch our minds around, and our minds know we're in the wrong body, but our memories didn't switch with us."

"Brilliant, John!" Sherlock exclaimed. John beamed.

"Terrific," Lestrade said. "So how can we be sure it's that and not that you're brain damaged before we strap you back in to switch you back?"

John and Sherlock shared another silent conversation.

"Because brain damage would have shown up in the brain scans?" Lestrade might have been convinced if John, who switched or not at least remembered studying medicine, had been the one to speak. As it was, he was mostly just weirded out by how uncertain Sherlock sounded. It wasn't the sort of tone Sherlock normally allowed.

"Sure," John agreed confidently, which would have been more convincing if he hadn't shared another silent conversation with Sherlock again first.

Then, before Lestrade could point out just how stupid it was to mess around with their own brains using a madman's invention without a good deal more research into its safety, both men were heading for the door. Once again, John was striding confidently ahead and Sherlock, who had the advantage of long legs and, apparently, still retained the ability to make his coat billow in an impressive manner, nonetheless was once again half a step behind.

Habit had Sherlock waving a hand to flag down a cab. No cabs stopped.

Lestrade had just enough time to jump into his own car to follow the cab that had been called over by John.

**6**

Lestrade got a text on the way, though he had to wait until a stoplight to read it. Donovan had indeed pressed charges and wanted Lestrade to haul Sherlock in to be locked up. Just another thing he'd have to sort out when this was all over. With any luck, they wouldn't run into her on the way to inspect the machine. Inspect it, because he was not going to just allow his civilian consultants to strap themselves into it just on the off chance it would fix them.

That isn't how things went. They weren't arrested by Donovan. Nor did they fry themselves with questionable technology. Somewhere along the way, quite likely while Lestrade was distracted by his text, the cab managed to slip away and, for reasons unknown, had taken the two men to somewhere that was not Scotland Yard.

Lestrade made it to the machine only to discover he had lost his consultants.

**7**

Lestrade might have lost them, but John and Sherlock knew exactly where they were. They were in a warehouse near the Thames, tied uncomfortably to chairs, while a consulting criminal gloated at them.

"I will admit this wasn't the effect I was hoping for," Moriarty said, not sounding disappointed in the least, "I was hoping to get to body hop. Can you imagine what I could do, changing bodies. Why, I could have been John for a day, gotten blood all over his hands, then left him to do the jail time. Or I could play at being Sherly. Oh, can you imagine? I could take down the British government, in a body like that. Or maybe just play with myself a bit. Do you think it counts as masturbation, if it's your body but someone else's mind? The fun I could have had!"

"If you touch John," Sherlock said, his voice ice cold, "I will dissect you in a freezer just to ensure you live longer while I rip your intestines out through your throat."

The sheer calculated malice in his voice actually gave Moriarty pause for a moment. Unfortunately, in the next he actually appeared delighted.

"Don't you mean, if I touch Sherlock?" he asked, "Him being you and you being him." He cocked his head to the side thoughtfully, eyes looking Sherlock up and down. "Do you know, I think I actually like you better as John. Same genius brain, twice the ruthlessness. The things we could do together!"

"I think you'll find we're both quite ruthless," John said. "Do you know how long a doctor can keep a body alive despite all the pain it's going through?"

"Do shut up, Sherlockjohn," Moriarty answered. "I want to play with Johnsherlock now. Your new body has left you a tad…ordinary in the brain."

"Do you know," Sherlock said, "I have all these memories. I remember enjoying our battle. I remember being a bit sad about how it inevitably had to end. It's not that I ever liked you, or even admired you. It was that you were a proper challenge, and it was hard to let that go. I remember that."

And then Sherlock wasn't sitting tied in his chair anymore, because his brain and his muscles also remembered how to slip free. And Sherlock who wasn't exactly Sherlock and who most definitely wasn't playing a game, calmly and with the precision of a doctor, removed the disease that was Moriarty from mortal existence.

"You killed him," John said, staring.

"Do you know," Sherlock said, "That's actually the first person I've killed, directly at least. I think a few criminals I got in fights with might have ended up with fatal wounds, but that wasn't actually on purpose. At least, that's what this memory tells me."

"Are you…alright?" John asked, still staring at Moriarty's body. He didn't feel sickened, or sad. He also didn't feel elated or even relieved. John remembered killing people himself. He still felt something that wasn't quite pleasant.

"I suppose he didn't expect that," Sherlock said, "Or we'd have been killed ourselves by now. He actually seems to have come alone." Then with the same calmness he had used to deliver death, he released John from his chair. John stayed seated anyway, still staring at Moriarty's body.

"Do you suppose," Sherlock said, "That you got so angry about Sally calling me a freak because it was really you that she was calling a freak, since we got switched?"

"No," John answered. "I think…the one thing I've been certain of since this whole thing is that I care about you. You, you, not your body. My body. Whatever. You did say I was your only friend. I mean my body said that when I was me. Though it's not true, at least it doesn't seem true from this body's memories. I remember caring about you, and talking to Greg, and he definitely cares about you. A bit like a son, but more like a brother, I think."

"I remember caring about you," Sherlock said. "Sometimes this body cared almost too much, I think. It got used to not caring, you see. Because no one ever cared back who didn't have to, like family."

There was a longer silence as they tried to unravel their own thoughts from their body's memories.

"I suppose we should get switched back," John said at last. "This just feels too…wrong."

"We'll have to call Mycroft," Sherlock said, looking down at Moriarty with a thoughtful frown. "Do you think this means I really am a phsychopath? The John me, I mean. You never killed on purpose before. I don't remember particularly caring when strangers die, but I…your body, it never went out to kill them."

"You're not a psychopath," John answered vehemently. "He was a psychopath. Maybe if I had had a bit more you in me, he would have done less harm to the world."

Sherlock searched Moriarty's pockets and found his phone. He called Mycroft. He could remember preferring to text but just then he did not feel disinclined to call.

**8**

"Thanks for telling me you're alive, you wankers," Lestrade said when they finally joined him at Scotland Yard. Mycroft had been entirely too fascinated by John as Sherlock and Sherlock as John. They had tried to hide their condition, of course, but if even Lestrade could tell that something was off, Mycroft had known just from the phone call. Not specifically what had happened of course, but enough to be suspicious.

"Sorry," said Sherlock, and then because he suspected his body wouldn't be inclined to say those words in a few minutes, "I really do care about you, you know. This body, I mean, Sherlock's body. And I respect your work. I just spent too long fighting the world, I didn't know how to stop."

Lestrade stared at him.

"For what it's worth, this body respects you too and enjoys our pub nights," John announced. "To be honest, I doubt I'll be so inclined to tell you either; I remember avoiding talking about emotions just as much as I probably didn't understand them when I was Sherlock."

"Ok, this is getting too weird," Lestrade said. "Just…are you certain this is safe?"

"As safe as anything we do," Sherlock answered.

They strapped the odd metal bowls to their heads. This time they didn't bother with the arm restraints.

"Pull the lever!" John exclaimed, cackling as gleefully as Dr. Frankenstein might.

Lestrade hesitated. Donovan, who had finally caught up to them only to find them hooked up to the weird machine, did not.

There were weird flashing lights. Both men lost consciousness. Again.

**9**

William Sherlock Scott Holmes and John Hamish Watson opened their eyes. They felt…right.

"I…you…I killed Moriarty," Sherlock said.

"Well," John answered, "He was a very bad man."

"Psychopaths!" Donovan muttered. "Are you back to normal now?"

"If you want to arrest me, you should really be arresting John," Sherlock said.

"Hey!" John said.

"You really are much more violent than people give you credit for," Sherlock insisted, sounding rather fond as he said it. As though that were a good thing.

"And you really are much kinder than people give you credit for," John said. "It's the lack of proper compassion that throws people. And the way you occasionally throw me on the law while you run off."

"I'd have gotten you out again!" Sherlock answered indignantly.

"Just tell us that you're both you again," Lestrade insisted. "And we didn't just make brain damage worse."

"I know one way to find out," Sherlock decided. "Come along, John!" And he spun about, making his coat flair up, before striding with long, confident steps for the door. John followed a step or two behind. Lestrade and Donovan looked at each other.

"You are going to explain what this has all been about, right?" she demanded.

"It's complicated," Lestrade said, as they both moved to catch up to the two men. They had to jog a bit. Sherlock had put his long legs to good use. "The machine actually did switch their minds. It just didn't switch their memories, took so it took a while for anyone to notice." Okay, maybe it wasn't that complicated.

They caught up to Sherlock and John just in time for Sherlock to order John to call a cab. With a shrug, John waved for one. It didn't stop. Then Sherlock stepped forward, raised his hand, and no less than two different cabs swerved towards them.

"Thank goodness," Lestrade said. "They're back."

Of course, that was when John and Sherlock made their escape in the first cap Sherlock had called, leaving Lestrade to deal with an incensed Donovan.

A week later, when everything had calmed down, Lestrade finally called them in for a new case.

Sherlock solved it as quickly as ever. He was excited and graceful and in no way blended in with the background. He was very enthusiastic at the chance to study a woman's mutilated corpse.

He managed to fire out his brilliant observations without putting down a single person present. Perhaps he had actually grown from the experience of switching with John? Then, just as Lestrade had moved away to send someone off to pick up a potential suspect based off of evidence Sherlock had found, Sherlock's voice rose in volume, as acerbic as ever.

"You imbecile, Anderson! Seriously, I think I'm losing brain cells as we speak! Your footprints have trampled all over the marks here!"

The more things change, Lestrade thought. The more things change. And then John brutally tackled the would-be murderer who burst from the secret room Anderson had been standing in front of.

Lestrade cuffed the suspect. Life went on.


End file.
